The invention relates to a diffractive security element as set forth in the classifying portion of claim 1.
Such diffractive security elements are used for verifying the authenticity of a document and are distinguished by an optically variable pattern which changes in a striking and predetermined manner from the point of view of the person observing it by virtue of rotation or tilting movement.
Diffractive security elements of that known from many sources, reference is made here as representative examples to EP 0 105 099 B1, EP 0 330 738 B1 and EP 0 375 833 B1. They are distinguished by the brilliance of the patterns and the movement effect in the pattern, they are embedded in a thin laminate of plastic material and they are glued in the form of a stamp on to documents such as banknotes, bonds, personal identity papers, passports, visas, identity cards and so forth. Materials which can be used for production of the security elements are summarised in EP 0 201 323 B1.
Modern photocopiers and scanner devices are capable of duplicating such a document in apparently true colors. The diffractive security elements are also copied, in which case admittedly the brilliance and the movement effect are lost so that the pattern which is visible in the original at a single predetermined angle of view is reproduced as an image with the printing colors of a color photocopier. Such copies of documents can be easily confused with the original under poor lighting conditions or if the observer is not paying attention. The known security elements suffer from the disadvantage that the man in the street cannot easily recognise the copies as being such.
It is known from EP 0 490 457 B1 that it is possible to dispose in a visually recognisable image a second, visually unrecognisable image comprising fine line portions. The content of the second image is coded in the slope of the line portions with respect to the line portions of the background. In the copying operation the second image appears over the first image with a blackening effect which is dependent on the slope angles of the line portions. Therefore the second image is dependent on the position of the original on the copier machine. Theoretical considerations in that respect are set forth in xe2x80x98Optical Document Securityxe2x80x99, van Renesse, Editor, ISDN No 0-89006-982-4, pages 127-148.
The object of the present invention is to provide a visually recognisable, inexpensive diffractive security element having an optically variable surface pattern which, in a copy produced by a color photocopier, has second concealed information which is independent of the surface pattern and which is not visible to the naked eye.
In accordance with the invention the specified object is attained by the features recited in the characterising portion of claim 1. Advantageous configurations of the invention are set forth in the appendant claims.